The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which upon heating decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon produce is called "shale oil".
A number of methods have been proposed for processing the oil shale which involve either first mining the kerogen bearing shale and processing the shale on the surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972, to Donald E. Garrett, assigned to the assignee of this application and incorporated herin by this reference. This patent describes in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale by fragmenting such formation to form a stationary, fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Hot retorting gases are passes through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale.
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and introduction of an oxygen containing retort inlet mixture downwardly into the retort as an oxygen supplying gaseous combustion zone feed to advance the combustion zone downwardly through the retort. In the combustion zone oxygen in the combustion zone feed is depleted by reaction with hot carbonaceous materials to produce heat and combustion gas. By the continued introduction of the retort inlet mixture downwardly into the retort, the combustion zone is advanced downwardly through the retort.
The combustion gas and the portion of the combustion zone feed that does not take part in the combustion process pass through the fragmented mass in the retort on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid products including gaseous and liquid hydrocarbon products and to a residual solid carbonaceous material.
The liquid products and gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, are collected at the bottom of the retort. An off gas containing combustion gas generated in the combustion zone, gaseous products produced in the retorting zone, gas from carbonated decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process is also withdrawn from the bottom of the retort. The products of retorting are referred to herein as liquid and gaseous products.
The residual carbonaceous material in the retorted oil shale can be used as fuel for advancing the combustion zone through the retorted oil shale. When the residual carbonaceous material is heated to its spontaneous ignition temperature it reacts with oxygen. As the residual carbonaceous material becomes depleted in the combustion process, the oxygen penetrates further into the oil shale retort where it combines with remaining unoxidized residual carbonaceous material, thereby causing the combustion zone to advace through the fragmented oil shale.
The rate of retorting of the oil shale to liquid and gaseous products is temperature dependent, with relatively slow retorting occurring at 600.degree. F., and relatively rapid retorting of the kerogen in oil shale occurring at 950.degree. F. and higher temperatures. As the retorting of a segment of the fragmented oil shale in the retorting zone progresses and less heat is extracted from the gases passing through the segment, the combustion gas heats the oil shale farther on the advancing side of the combustion zone to retorting temperatures, thus advancing the retorting zone on the advancing side of the combustion zone.
U.S. patent application Ser. No. 790,350 filed on Apr. 25, 1977, by Ned M. Hutchins, assigned to the assignee of this application, and incorporated herein by this reference, describes a method of forming an in situ oil shale retort in a subterranean formation containing oil shale. According to the '350 patent application, a first portion of the subterranean formation is excavated at a working level to form an open base of operation at an elevation in the formation above the top boundary of the fragmented mass being formed. A second portion of the formation is excavated for forming at least one void within the boundaries of the fragmented mass being formed. A third portion of the formation is expanded toward such a void to form a fragmented permeable mass of particles containing oil shale and to leave a horizontal sill pillar of unfragmented formation between the top of the fragmented mass and the bottom of the base of operation. As used herein, the term "horizontal sill pillar" refers to unfragmented formation between a working level and the top boundary of a fragmented mass. The term "working level" refers to the general elevation in a subterranean formation at which underground workings or galleries are excavated and utilized in the formation of a fragmented mass below a horizontal sill pillar in a retort being formed. Underground workings include excavations of any desired configuration, such as drifts, adits, tunnels, cross-cuts, rooms or the like.
In preparation of a retort by a method such as described in the '350 application, a portion of the top of the fragmented permeable mass can be separated from the top boundary of unfragmented formation by a substantially empty pocket or void. The presence of such a pocket can be a very serious problem during retorting of oil shale in the fragmented mass, because if unfragmented formation above the pocket is heated to a temperature of about 400.degree. to 500.degree. F. or higher, such unfragmented formation can slough into the pocket.
Sloughing of formation into such a pocket is very undesirable. Such sloughing can jeopardize the stability of the overlying formation. If the top boundary of the fragmented mass is the bottom of a sill pillar, and if excessive thermal sloughing occurs, the horizontal sill pilar could be weakened, therby damaging the base of operation.
Another very undesirable effect of thermal sloughing is that unfragmented formation sloughing into a pocket can cause difficulty in start-up due to cold raw oil shale being added to the rubble pile. Moreover, if a large quantity of overlying formation sloughs into a pocket, the amount of fuel required for start-up may be greatly increased.
There is a need for a method for retorting oil shale below a pocket in a fragmented permeable mass, and for establishing a combustion zone in the fragmented mass below such a pocket.